The country's biggest company could wean itself off coal and phase in sustainable wood fuels to power its dairy plants, an industry expert says.

John Gifford made the argument yesterday during day seven of the resource-consent hearing that will decide whether Glencoal, a subsidiary of Fonterra, can develop an opencast coalmine at Mangatangi, in North Waikato.

The Coal Action Network Aotearoa was behind his appearance and, while it could not be mentioned in evidence, it wants coal use phased out because of its contribution to climate change.

A total of 700,000 tonnes of coal is to be extracted from the mine at a rate of 120,000 tonnes a year, if it gets the go-ahead. It will be burned at the company's dairy factories in Waitoa, Te Awamutu and Hautapu.

Mr Gifford said there was no question that the transition from coal to wood could be done. "Ultimately, you'd hopefully get to the point where you build up a supply chain, bring down the costs and the whole wood-fuel situation completely substitutes for the coal. And the greenhouse gas emissions arising from that are completely eliminated," he said.

Glencoal Energy and Fonterra would not comment on the proposition during the hearing.

However, their expert witness Mike Suggate spoke to the hearing about the use of wood fuels as a substitution for coal, and Mr Gifford agreed with the points he made: that the conversion to wood was technically feasible; that the price of wood fuels was unlikely to rise, unlike coal; that adequate wood fuel was likely to be available to meet the demand, even for full replacement of coal; and that the extensive work needed to modify the boilers, secure wood fuel and establish the supply chain could potentially be completed by August/September 2016, allowing "substantial" use of wood fuels (though, Mr Gifford believes that would be a challenge).

Mr Gifford, who has more than 30 years' experience in the forestry, bioenergy and environmental sectors, said the option of using woody fuels, to avoid coalmining, was "potentially viable" for Fonterra. But it might be at "a short-term premium".

Wood could be sourced from existing forestry activities in the area over the short-to-medium term, he said. Estimates indicated that within about 110km [of Morrinsville], 335,622 green tonnes of wood fuel were available.